1. Field of the Invention
The present invention relates to a dispersion-compensating fiber which is applied to an optical transmission line including a 1.3-xcexcm zero-dispersion single-mode optical fiber and improves transmission characteristics of the whole optical transmission line with respect to light in a 1.55-xcexcm wavelength band.
2. Related Background Art
In optical fiber transmission line networks used for high-speed, large-capacity communications over a long distance, the dispersion (chromatic dispersion) expressed by the sum of the material dispersion (dispersion caused by the wavelength dependence of refractive index inherent in the material of the optical fiber) and structural dispersion (dispersion caused by the wavelength dependence of the group velocity in the propagation mode) in the single-mode optical fiber employed as their transmission medium is a limiting factor for the transmission capacity. Namely, even when light outputted from a light source is assumed to have a single wavelength, it has a spectrum width in the strict sense. When such an optical pulse propagates through a single-mode optical fiber having a predetermined chromatic dispersion characteristic, the pulse form may collapse since the propagation velocity varies among definite spectral components. This dispersion is expressed by a unit (ps/km/nm) of propagation delay time difference per unit spectrum width (nm) and unit optical fiber length (km). Also, it has been known that the material dispersion and structural dispersion cancel each other in the single-mode optical fiber, so that the dispersion becomes zero in the vicinity of 1.31 xcexcm.
As means for compensating for the dispersion of the single-mode optical fiber, a dispersion-compensating fiber is used in general. In particular, since the transmission loss of optical fibers becomes the lowest in the 1.55-xcexcm wavelength band, it has been desired that optical communications be carried out by use of light in the 1.55-xcexcm wavelength band. On the other hand, a number of single-mode optical fibers having a zero-dispersion wavelength in the vicinity of 1.3 xcexcm have been laid so far, and hence there are needs for carrying out optical communications in the 1.55-xcexcm wavelength band by utilizing such existing single-mode optical fiber transmission line networks. In such a case, when a dispersion-compensating fiber having a negative dispersion in the 1.55-xcexcm wavelength band is connected to a single-mode optical fiber having a positive dispersion in the 1.55-xcexcm wavelength band, the chromatic dispersion of the whole optical transmission line can be compensated for. For example, Japanese Patent Application Laid-Open No. HEI 8-136758 and Japanese Patent Application Laid-Open No. HEI 8-313750 disclose dispersion-compensating fibers comprising a double cladding structure. On the other hand, Japanese Patent Application Laid-Open No. HEI 6-11620 discloses a dispersion-compensating fiber comprising a triple cladding structure.
The inventors have studied the above-mentioned prior art and, as a result, have found problems as follows. Namely, in an optical transmission line constituted by a single-mode optical fiber and a dispersion-compensating fiber for compensating for the chromatic dispersion of the single-mode optical fiber, chromatic dispersion cannot be all prevented from occurring in wavelengths of the wavelength band in use, but it occurs at least in a wavelength band deviated from the vicinity of the zero-dispersion wavelength. Consequently, if various characteristics of the conventional dispersion-compensating fiber are not sufficiently optimized for compensating for the chromatic dispersion of a single-mode optical fiber having a zero-dispersion wavelength near 1.3 xcexcm (hereinafter referred to as 1.3SMF), various transmission characteristics may be caused to fluctuate among wavelengths in optical communications of wavelength division multiplexing (WDM) system, in which different wavelengths of signal light are multiplexed so as to enable the transmission capacity to further enhance, and the like.
Here, dispersion slope is given by the gradient of a graph which indicates chromatic dispersion, and is expressed by a unit of ps/nm2/km.
In order to overcome such problems as those mentioned above, it is an object of the present invention to provide a dispersion-compensating fiber comprising a structure which compensates for the chromatic dispersion of a 1.3SMF, improves, when applied to an optical transmission system including the 1.3SMF, the total dispersion slope in the optical transmission system as a whole in a wider wavelength band, and enables dispersion to be compensated for efficiently and its size to become smaller.
The dispersion-compensating fiber according to the present invention is an optical fiber for compensating for the chromatic dispersion of a 1.3SMF, and comprises a core region extending along a predetermined reference axis and a cladding region provided on the outer periphery of the core region. The core region is constituted by a core having an outer diameter 2a. The cladding region is constituted by a first cladding, provided on the outer periphery of the core, having an outer diameter 2b; a second cladding, provided on the outer periphery of the first cladding, having an outer diameter 2c, and a third cladding provided on the outer periphery of the second cladding.
In this dispersion-compensating fiber, the refractive index n1 of the core, the refractive index n2 of the first cladding, the refractive index n3 of the second cladding, and the refractive index n4 of the third cladding satisfy the condition of n1 greater than n3 greater than n4 greater than n2. Also, the relative refractive index difference xcex941 of the core with respect to the third cladding and the relative refractive index difference xcex942 of the first cladding with respect to the third cladding satisfy at least the conditions of 1%xe2x89xa6xcex941xe2x89xa63%, and xcex942xe2x89xa6xe2x88x920.3%. Further, in this dispersion-compensating fiber, the outer diameter 2a of the core and the outer diameter 2c of the second cladding satisfy the condition of 2a/2cxe2x89xa60.3.
The dispersion-compensating fiber according to the present invention is characterized in that, when constituting an optical transmission system together with a 1.3SMF, it has a length sufficient for the optical transmission system to yield a total dispersion slope of xe2x88x920.024 ps/nm2/km or more but 0.024 ps/nm2/km or less with respect to respective light components having a shortest wavelength xcexS and a longest wavelength xcexL in signal wavelengths within the wavelength band in use.
Specifically, the dispersion-compensating fiber has a length LDCF which is set so as to satisfy the following condition with respect to light having a wavelength xcexm in signal wavelengths within the wavelength band in use:
|DSMF(xcexm)xc2x7LSMF+DDCF(xcexm)xc2x7LDCF|xe2x89xa6200 ps/nm
where
DSMF(xcexm) is the dispersion of the 1.3SMF at the wavelength xcexm;
LSMF is the length of the 1.3SMF;
DDCF(xcexm) is the dispersion of the dispersion-compensating fiber at the wavelength xcexm; and
LDCF is the length of the dispersion-compensating fiber.
More preferably, the length LDCF of the dispersion-compensating fiber is set so as to satisfy the following condition with respect to all signal wavelengths xcexall of light within the wavelength band in use:
|DSMF(xcexall)xc2x7LSMF+DDCF(xcexall)xc2x7LDCF|xe2x89xa6200 ps/ nm
where
DSMF(xcexall) is the dispersion of the 1.3SMF at all the wavelengths xcexall in use;
LSMF is the length of the 1.3SMF;
DDCF(xcexall) is the dispersion of the dispersion-compensating fiber at all the wavelengths xcexall in use; and
LDCF is the length of the dispersion-compensating fiber.
Thus, the dispersion-compensating fiber comprising a core region having a single core and a cladding region, provided on the outer periphery of the core region, having three claddings (a triple cladding structure) can improve the total dispersion slope in the optical transmission system to which the dispersion-compensating fiber is applied, if the refractive index and outer diameter of each part thereof are set so as to satisfy such conditions as those mentioned above. Also, when the dispersion-compensating fiber is wound like a coil so as to constitute a module, for example, it is possible to select a chromatic dispersion having an absolute value which is large enough to enable the dispersion to be compensated for efficiently and the apparatus to become smaller.
In particular, when a module employing a double cladding structure realizing an improvement in total dispersion slope is to be made smaller, the shortening of the optical fiber and the reduction of the bending loss occurring upon being wound like a coil cannot be achieved at the same time. Such a problem can be overcome when various characteristics of the optical fiber having the triple cladding structure as mentioned above are optimized.
For example, in the dispersion-compensating fiber having a triple cladding structure, the length of the dispersion-compensating fiber necessary for dispersion compensation can be made shorter as the absolute value of the dispersion having a negative value is greater, whereby the dispersion compensation can be made more efficient. In particular, when the relative refractive index difference xcex941 of the core with respect to the third cladding is set within the range of 1% or more but 3% or less, dispersion can be compensated for at a high efficiency.
Though the dispersion increases as the value of xcex941 is enhanced, it is necessary that, for example, the amount of GeO2 added to the core be increased in order to enhance the value of xcex941. On the other hand, an increase in the amount of addition of GeO2 causes the transmission loss to increase. Therefore, it is preferred that, within the range of condition concerning xcex941, the range of xcex941 be appropriately selected, in view of various conditions such as use, equipment, and the like, such that the value of xcex941 is lowered (e.g., 1%xe2x89xa6xcex941xe2x89xa62%) in the case where a lower transmission loss is required and that the value of xcex941 is enhanced (e.g., 2%xe2x89xa6xcex941xe2x89xa63%) in the case where a higher dispersion compensation efficiency due to a higher dispersion is required.
Here, in the dispersion-compensating fiber according to the present invention, the relative refractive index difference xcex943 of the second cladding with respect to the third cladding preferably satisfies the condition of 0% less than xcex943xe2x89xa60.1%. Also, the ratio of the outer diameter 2b of the first cladding to the outer diameter 2a of the core preferably satisfies the condition of 2.5xe2x89xa62b/2axe2x89xa63.5. In addition, with respect to light having a wavelength of 1.55 xcexcm, the dispersion-compensating fiber according to the present invention has a bending loss of 0.1 dB/m or less at a diameter of 60 mm, a polarization mode dispersion of 0.5 psxc2x7kmxe2x88x92xc2xd or less, and a transmission loss of 1 dB/km or less.
In practice, there is a case where such a dispersion-compensating fiber is employed as a small-size module wound like a coil. In this case, in particular, lowering the bending loss enables the apparatus to reduce its size and suppress its resultant increase in transmission loss.
When an optical transmission system is constituted by a 1.3SMF and the dispersion-compensating fiber having a length sufficient for compensating for the dispersion of the 1.3SMF as mentioned in the foregoing, this optical transmission system yields a total dispersion slope of xe2x88x920.024 ps/nm2/km or more but 0.024 ps/nm2/km or less, preferably xe2x88x920.012 ps/nm2/km or more but 0.012 ps/nm2/km or less, with respect to respective light components having the shortest wavelength xcexs and the longest wavelength xcexL in signal wavelengths within the wavelength band in use.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.